Nuclear magnetic resonance (NMR) is a powerful and well established tool for studying samples and sample interactions. It can also be used for imaging, for example, Magnetic Resonance Imaging (MRI).
In NMR, the spin and magnetism of atomic nuclei are exploited to provide information about the chemical composition, spatial distribution, or molecular motion of the molecules or atoms.
One of the limitations of NMR is the low intrinsic signal strength. Attempts to address this problem have included nuclear spin hyperpolarization experiments, for example using parahydrogen (C. R. Bowers and D. P. Weitekamp, Phys. Rev. Lett. 57, 2645-2648 (1986); C. R. Bowers and D. P. Weitekamp, J. Am. Chem. Soc. 109, 5541-5542 (1987); M. G. Pravica and D. P. Weitekamp, Chem. Phys. Lett. 145, 255-258 (1988)).
One of the principal factors contributing to the applicability of NMR based techniques is the relatively long lifetime of the nuclear spin order. This long lifetime allows NMR to be used to follow processes such as diffusion, flow, slow molecular motion and chemical reactions. The relaxation of the nuclear spins back to thermal equilibrium, and hence the lifetime of the nuclear spin memory is characterised by a time constant, T1 known as the longitudinal relaxation time constant or as the spin lattice relaxation time constant. The time constant T1 is governed by molecular motion, and is in general different from the phase memory time T2, which depends upon molecular motion and also external perturbations such as magnetic field gradients. In general, T1 is always longer than 0.5 T2, although in common situations, T1 is much greater than T2, often by an order of magnitude.
Generally, for many samples, the time constant T1 is of the order of seconds. Thus, the time for following a process of the type mentioned above is very limited.
Attempts have been made to extend the phase memory time in the context of quantum computation, using the concept of Decoherence-Free Subspaces (L. Viola, E. M. Fortunato, M. A. Pravia, E. Knill, R. Laflamme and D. G. Cory, Science 293, 2059-2063 (2001); E. M. Fortunato, L. Viola, M. A. Pravia, E. Knill, R. Laflamme, T. F. Havel and D. G. Cory, quant-ph/0210057 (2002); D. A. Lidar and K. B. Whaley, quant-ph/0301032 (2003); J. E. Ollerenshaw, D. A. Lidar and L. E. Kay, quant-ph/0302175 (2003)). However, although the compensation of artificial noise contributions to the phase memory time T2 has been demonstrated, the extension of the spin memory time beyond T1 has not been demonstrated.